1. Field of the Invention
This invention relates to a rewritable optical information recording medium which can be used for writing, erasing of recorded data and rerecording with controlled laser light.
2. Description of the Prior Art
Photochromic materials are known as a kind of material capable of changing its color in a reversible manner. Photochromic materials are used, for example, for optical recording mediums as described in the report of the investigation of optical disk memory materials, March 1982, Nippon Denshi Kogyo Shinko Kyokai, 182.
The use of this kind of optical recording medium will be briefly described below. Laser light having a wavelength .lambda..sub.a is condensed to a recording layer which is formed on a surface of a base and which contains a photochromic coloring matter in a color-extinguished state (color-extinguished seed). A photochromic reaction of the coloring matter is thereby brought about at the spot irradiated with the laser light, and the coloring matter is changed into a color-developed seed having a maximum absorption at a wavelength .lambda..sub.b, thereby recording information. To read out the recorded information, the recording layer is irradiated with laser light having the wavelength .lambda..sub.b but weak enough to avoid occurrence of a photochromic reaction, and the intensity of transmission light or reflection light thereby obtained is detected.
Strong laser light having the wavelength .lambda..sub.b may be used for the same operation as writing to erase the written information by changing the color-developed seed into a color-extinguished seed, thereby enabling rewriting of information.
However, ordinary photochromic coloring matters are very unstable in colored states and spontaneously discolor easily even during storage in a dark place. For this reason, optical information recording mediums in which photochromic coloring matters are used are not capable of maintaining information for a long period of time.
A method for solving this problem has been tried in which an antioxidant for preventing oxidation of plastics, e.g., a phenolic antioxidant, is added to the coloring matter. However, the addition of a phenolic antioxidant entails some drawbacks. That is, the coloring density (optical density) of the photochromic coloring matter is reduced and the wavelength of the maximum absorption in the color-developed seed (colored state) is shifted to the short wavelength side. In particular, it is disadvantageous to shift the maximum absorption wavelength of the color-developed seed to the short wavelength side where the photochromic material is used as a medium for both recording and reproduction using a semiconductor laser.
Japanese Patent Laid-Open No. 62-147453 discloses that an optical recording medium capable of being stably maintained in a color-developed state can be obtained by using a mixture of a photochromic spiropyran compound and a compound having a polarity or, more specifically, a carboxylic acid.